Al-based plated steel sheet

ABSTRACT

An Al-plated steel sheet includes: a base; an Al-plating layer formed on at least one of opposite surfaces of the base; and a surface layer formed on the Al-plating layer, the surface layer containing: ZnO particles; an organic resin; and acetylacetonato in an amount in a range from 10 mass % to 30 mass %, both inclusive, with respect to a total mass of the surface layer. A mean particle size of the ZnO particles is in a range from 0.10 μm to 5.00 μm, both inclusive, and an amount of coating of the ZnO particles is in a range from 0.5 g/m2 to 10.0 g/m2, both inclusive, in terms of metal Zn.

TECHNICAL FIELD

The present invention relates to an Al-plated steel sheet suitable forhot pressing capable of exhibiting sufficient formability (lubricity),corrosion resistance (corrosion resistance of painted steel), and thelike during hot pressing.

BACKGROUND ART

There has recently been an increasing demand for reducing consumption offossil fuel for the purpose of environmental protection and globalwarming reduction. Such a demand has affected a variety of manufacturingindustries. The automobile industry, which provides transportationdevices necessary for daily life and the like, has also been affected,so that fuel consumption has been required to be reduced by, forinstance, a reduction of the weight of a vehicle body. Many automobileparts are made of iron, especially, steel sheet. A reduction in a totalweight of steel sheet in use is thus important for a reduction in theweight of a vehicle body and, consequently, fuel consumption.

However, simply reducing a thickness of steel sheet is not permitted dueto the necessity to ensure the safety of automobiles, so that amechanical strength of the steel sheet needs to be maintained. Such ademand for steel sheet has been increased not only in the automobileindustry but also in a variety of other manufacturing industries.Accordingly, studies and developments have been conducted on anenhancement of the mechanical strength of steel sheet to provide steelsheet whose mechanical strength can be maintained or improved even whenit is thinned as compared with a typical steel sheet.

A typical material with an excellent mechanical strength tends to havelowered formability and shape freezing properties during a formingprocess such as bending, so that the material is difficult to form intoa complicated shape. Technologies capable of overcoming such aformability problem include so-called hot pressing (also referred to ashot stamping, hot pressing, die quenching, and press hardening). In hotpressing, a material (target to be subjected to a forming process) isfirst heated to a high temperature (austenite range), and cooled afterthe steel sheet softened by heating is subjected to press forming.

Such hot pressing, in which a material is first heated to a hightemperature to be softened, allows the material to be easilypress-formed. Additionally, a quenching effect provided by cooling afterthe forming increases the mechanical strength of the material. The hotpressing can thus provide a molding article with good shape freezingproperties and high mechanical strength.

However, when steel sheet is heated to, for instance, 800 degrees C. orhigher for hot pressing of the steel sheet, scale (oxide) is generatedas a result of oxidization of an iron component in a surface of thesteel sheet. This necessitates removal of the scale (descaling) afterthe hot pressing, lowering the productivity. Furthermore, in producing amolding article designed to exhibit corrosion resistance, ananti-corrosion treatment has to be performed on a surface of the moldingarticle subsequently to hot pressing or, alternatively, surface cleaningor surface treatment has to be performed to form a metallic coating,further lowering the productivity.

Such a lowered productivity can be prevented by, for instance, forming ametallic coating on steel sheet. A typical metallic coating on steelsheet can be made of a variety of materials such as organic materialsand inorganic materials. Especially, a zinc-plated steel sheet capableof sacrificial protection of steel sheet has been widely used as steelsheet for automobiles and the like in terms of anti-corrosionperformance and steel sheet production technology (Patent Literature 1).

However, a heating temperature (700 degrees C. to 1000 degrees C.) forhot pressing is higher than decomposition temperatures for organicmaterials and boiling temperature of Zn (zinc). Thus, as steel sheet isheated for hot pressing, a plating layer on the surface of the steelsheet would be oxidized or evaporated, considerably impairing a surfacetexture and deteriorating slidability during the hot pressing. Toprevent such a problem, it is necessary to, for instance, add aluminumin a predetermined range of amount to the zinc-plating layer and,additionally, form a film configured to be melted at a press temperatureon the zinc-plating layer (Patent Literature 2).

Accordingly, for instance, an Al (aluminum) metallic coating, which ishigher in boiling temperature than an organic material film and a Znmetallic coating, is preferably formed on steel sheet to be heated to ahigh temperature for hot pressing to provide a so-called Al-plated steelsheet. Formation of such an Al metallic coating prevents adhesion ofscale onto a surface of steel sheet, eliminating the necessity of adescaling treatment or the like with improved productivity. The Almetallic coating also provides an anti-corrosion effect to improvecorrosion resistance of painted steel.

According to a proposed method, an Al-plated steel sheet with an Almetallic coating formed on steel of a predetermined steel composition issubjected to hot pressing (see, for instance, Patent Literature 3). Oneof the challenges of an Al-plating material for hot pressing asdisclosed in Patent Literature 3 is to improve the formability duringhot pressing. There are some problems regarding the formability duringhot pressing. For instance, an Fe—Al—Si plating layer generated duringheating bites into a mold due to the hardness thereof or cumulates onthe mold due to the large friction coefficient thereof. Such problemswould result in damages on a surface of a product, impairing theappearance quality.

According to a proposed method to solve the above problems, a coatinglayer containing zinc oxide (ZnO) is stuck on a plating surface (see,for instance, Patent Literature 4). Specifically, the method disclosedin Patent Literature 4 includes: sticking a coating layer onto a surfaceof steel sheet, the coating layer containing a binder consisting of aresin component, a silane coupler, or the like so that ZnO is preventedfrom falling off; and volatilizing an organic solvent component of thebinder at a temperature of 300 degrees C. to 500 degrees C. during hotpressing so that only ZnO remains. This method is said to allow voids tobe generated by combustion and evaporation of the organic solvent,causing ZnO to be in point-contact with the molding metal for improvedlubricity.

According to another proposed method, an oxide such as ZnO is used toimprove temperature-rise properties during in-furnace heating orinfrared heating, chemical convertibility after heat pressing, andcorrosion resistance of painted steel as well as lubricity during hotpressing (see, for instance, Patent Literatures 5 to 8).

CITATION LIST Patent Literature(s)

Patent Literature 1 JP 2010-242188 A

Patent Literature 2 JP 2011-32498 A

Patent Literature 3 JP 2000-38640 A

Patent Literature 4 International Publication No. WO 2009/131233

Patent Literature 5 JP 2012-92365 A

Patent Literature 6 JP 2013-227620 A

Patent Literature 7 JP 2013-221202 A

Patent Literature 8 International Publication No. WO 2014/181653

SUMMARY OF THE INVENTION Problem(s) to be Solved by the Invention

As disclosed in Patent Literatures 4 to 8, a surface layer containingZnO formed on an Al-plating improves the slidability during hotpressing. However, studies by the inventors have proven that when anAl-plated steel sheet is heated on a conveyor for preheating andtransportation prior to hot pressing, a ZnO metallic coating disappearsfrom a stacked portion of the steel sheet in contact with the conveyor,lowering the hot slidability of this portion and, consequently, makingthe formability (lubricity) insufficient as a whole.

In view of the above, an object of the invention is to provide anAl-plated steel sheet suitable for hot pressing capable of reducing lossof ZnO from a contact area with a conveyor during heating to providesufficient formability (lubricity) during hot pressing, corrosionresistance (corrosion resistance of painted steel), and the like.

Means for Solving the Problem(s)

To solve the above problems, the inventors first researched the reasonsfor disappearance of the ZnO metallic coating at the stacked portion incontact with the conveyor. During continuous heating in an airatmosphere furnace, an organic resin component or Al present in aplating surface is usually combined with oxygen in atmosphere to beoxidized. However, the stacked portion of the plated steel sheet incontact with the conveyor is supplied with less oxygen, so that ZnO isreduced by Al to be oxidized. In other words, it has been found that ZnOis reduced to metal Zn as a result of adverse response of oxidation ofthe organic resin component or Al present in the plating surface and,consequently, the metal Zn disappears by vaporization.

Under such conditions, the inventors have also found that a surfacelayer formed on an Al-plating layer on the steel surface can reduce lossof ZnO at the contact area with the conveyor during heating and,consequently, provide sufficient formability during hot pressing,corrosion resistance, and the like, the surface layer containing ZnOparticles, an organic resin (binder), and an Al-oxidizer (an oxidizersupplying oxygen to Al) for reducing loss of ZnO.

The inventors have made the invention based on the above findings. Theoutline of the invention is as follows.

[1] According to an aspect of the invention, an Al-plated steel sheetfor hot pressing includes:

a base;

an Al-plating layer formed on at least one of opposite surfaces of thebase; and

a surface layer formed on the Al-plating layer, the surface layercontaining:

ZnO particles;

an organic resin; and

acetylacetonato in an amount in a range from 10 mass % to 30 mass %,both inclusive, with respect to a total mass of the surface layer, inwhich

a mean particle size of the ZnO particles is in a range from 0.10 μm to5.00 μm, both inclusive, and

an amount of coating of the ZnO particles is in a range from 0.5 g/m² to10.0 g/m², both inclusive, in terms of metal Zn.

[2] In the above aspect, the Al-plating layer and the surface layer areformed on each of the opposite surfaces of the base.

The Al-plated steel sheet according to the above aspect is improved incomponents of the surface layer (outermost layer) and the amount ofcoating of the ZnO particles (a component of the surface layer). As aresult of the improvement, the Al-plated steel sheet according to theaspect achieves sufficient formability (lubricity) during hot pressing,corrosion resistance (corrosion resistance of painted steel), and thelike.

BRIEF DESCRIPTION OF DRAWING(S)

FIG. 1 schematically illustrates a cross section of an Al-plated steelsheet according to an exemplary embodiment with a surface provided withan Al-plating layer and a surface layer.

FIG. 2 schematically illustrates a cross section of the Al-plated steelsheet according to the exemplary embodiment with opposite surfaces eachprovided with the Al-plating layer and the surface layer.

DESCRIPTION OF EMBODIMENT(S)

Detailed description will be made below on an Al-plated steel sheet(hereinafter, occasionally simply referred to as “steel sheet”) suitablefor hot pressing according to an exemplary embodiment of the invention.It should be noted that the invention is by no means limited to theexemplary embodiment. Components according to the exemplary embodimentinclude components replaceable or easily conceivable for those skilledin the art or components substantially the same as such replaceable oreasily conceivable components. Furthermore, a variety of configurationsaccording to the exemplary embodiment may be combined by those skilledin the art as desired within a scope of obviousness.

FIG. 1 illustrates an exemplary Al-plated steel sheet according to theexemplary embodiment of the invention. An Al-plated steel sheet 100according to the exemplary embodiment of the invention includes a base101, an Al-plating layer 103 formed on a surface of the base 101, and asurface layer 107 formed on a surface of the Al-plating layer 103. Thesurface layer 107 contains acetylacetonato and ZnO particles 109 unitedby an organic resin 111. The Al-plating layer 103 and the surface layer107 may be formed on each of opposite surfaces of the base 101 (see FIG.2). Each layer will be described below in detail.

Al-Plated Steel Sheet 100 Base 101

The base 101 (a member for forming the Al-plating layer 103) for theAl-plated steel sheet 100 according to the exemplary embodiment is amember configured to exhibit an excellent mechanical strength (i.e., avariety of properties against mechanical deformation and destroy such astensile strength, yield point, extensibility, durability during adrawing process, hardness, impact value, endurance strength, and creepstrength) during hot pressing subsequent to formation of the platinglayer. For instance, a member added with C (carbon) or alloy element forenhancement of hardenability is used. As a result, an automobile partproduced by hot-pressing the Al-plated steel sheet 100, which isproduced by forming the Al-plating layer 103 and the surface layer 107as described later, exhibits an excellent mechanical strength.

In other words, the base 101 for the Al-plated steel sheet 100 accordingto the exemplary embodiment may be any typical member with an excellentmechanical strength. For instance, the base 101 may be a membercontaining, but not limited to, the following components.

For instance, the base 101 according to the exemplary embodimentcontains, in mass %, C: not less than 0.01% nor more than 0.5%, Si: 2.0%or less, Mn: not less than 0.01% nor more than 3.5%, P: 0.1% or less, S:0.05% or less, Al: not less than 0.001% nor more than 0.1%, and N: 0.01%or less. The base 101 may selectively further contain, in mass %, one ofor two or more of Ti: not less than 0.005% nor more than 0.1%, B: notless than 0.0003% nor more than 0.01%, Cr: not less than 0.01% nor morethan 1.0%, Ni: not less than 0.01% nor more than 5.0%, Mo: not less than0.005% nor more than 2.0%, and Cu: not less than 0.005% nor more than1.0%, in addition to elements such as W, V, Nb, and Sb. Furthermore, thebalance of the base 101 consists of Fe and inevitable impurities.Detailed description will be made below on components added to the base101. A unit % of each component means mass % throughout the descriptionbelow.

C: not less than 0.01% nor more than 0.5%

Carbon (C), which is inevitably present in steel, is contained to ensurea desired mechanical strength of the base 101. Since an excessivereduction in the content of C increases costs for smelting, the contentof C is preferably 0.01% or more. Furthermore, C contained in an amountof 0.1% or more eliminates the necessity of addition of a large amountof other alloy elements for improvement in the mechanical strength,since addition of C has a large effect in improvement in the strength.Meanwhile, C contained in an amount exceeding 0.5% would cause frequentmelt fracture, although the base 101 can be further hardened.Accordingly, the content of C is preferably in a range from 0.01% to0.5%, both inclusive, more preferably from 0.1% to 0.4%, both inclusive,in terms of improvement in the strength and prevention of melt fracture.It should be noted that the content of C is further preferably in arange from 0.15% to 0.35%, both inclusive.

Si: 2.0% or less

Silicon (Si), which is added as a deoxidizer or the like, is an elementinevitably contained in a steel-smelting process. However, an excessiveaddition of Si lowers ductility during hot rolling of a steelmanufacturing process and, consequently, degrades the resulting surfacetexture, so that the content of Si is preferably 2.0% or less.

Furthermore, since Si is a reinforcing element capable of improving themechanical strength of the base 101, Si may be added for the purpose ofensuring the desired mechanical strength as well as C. Si contained inan amount of less than 0.01% is less effective in improving thestrength, so that the mechanical strength is unlikely to be sufficientlyimproved. Meanwhile, since Si is an oxidizable element, Si contained inan amount exceeding 0.6% lowers wettability during Al-melt plating,possibly causing failure in plating. Accordingly, Si is preferably addedin an amount of 0.01% to 0.6%, both inclusive. It should be noted thatthe content of Si is further preferably in a range from 0.05% to 0.5%,both inclusive.

Mn: not less than 0.01% nor more than 3.5%

Manganese (Mn), which is added as a deoxidizer or the like, is anelement inevitably contained in a steel-smelting process. However, anexcessive addition of Mn impairs the uniformity of the quality due tosegregation of Mn during casting, excessively hardening the steel sheetand, consequently, lowering the ductility during hot rolling and coldrolling. The content of Mn is thus preferably 3.5% or less. Meanwhile,reducing the content of Mn to less than 0.01% increases processes andcosts, so that the content of Mn is preferably 0.01% or more.Accordingly, the content of Mn is preferably in a range from 0.01% to3.5%, both inclusive.

Furthermore, Mn is an element capable of enhancing the hardenabilitywhile being a reinforcing element for the base 101. Mn is also effectivein reducing the hot shortness associated with S (sulfur), which is oneof the inevitable impurities, to a lower level. Thus, Mn contained in anamount of 0.5% or more can improve the hardenability and reduce the hotshortness. Meanwhile, Mn contained in an amount exceeding 3% would lowerthe strength due to an excessive increase in residual γ phase.Accordingly, Mn is more preferably added in an amount of 0.5% to 3%,both inclusive. It should be noted that the content of Mn is furtherpreferably in a range from 1% to 2%, both inclusive.

P: 0.1% or less

Phosphorus (P), which is a solid-solution reinforcing element whilebeing an inevitably contained element, is capable of improving thestrength of the base 101 with relatively low costs. However, a lowerlimit of the content of P is preferably 0.001% in terms of economicsmelting limit. Meanwhile, P contained in an amount exceeding 0.1% wouldlower the toughness of the base 101. Accordingly, the content of P ispreferably in a range from 0.001% to 0.1%, both inclusive. It should benoted that the content of P is further preferably in a range from 0.01%to 0.08%, both inclusive.

S: 0.05% or less

Sulfur (S), which is an inevitably contained element, is present as aninclusion in the form of MnS in the base 101, functioning as theorigination of destroy to impair the ductility and toughness and,consequently, deteriorate the formability. Accordingly, a lower contentof S is more preferred, so that an upper limit is preferably 0.05%.Meanwhile, a reduction in the content of S possibly increasesmanufacturing costs, so that a lower limit of the content of S ispreferably 0.001%. It should be noted that the content of S is furtherpreferably in a range from 0.01% to 0.02%, both inclusive.

Al: not less than 0.001% nor more than 0.1%

Aluminum (Al) is an element that impairs a plating performance whilebeing a component contained as a deoxidizer in the base 101.Accordingly, an upper limit of the content of Al is preferably 0.1%.Meanwhile, a lower limit of the content of Al is not limited but ispreferably, for instance, 0.001% in terms of economic smelting limit. Itshould be noted that the content of Al is further preferably in a rangefrom 0.01% to 0.08%, both inclusive.

N: 0.01% or less

Nitrogen (N), which is an inevitably contained element, is preferablycontained in a fixed amount in terms of stabilization of a variety ofproperties of the base 101. Specifically, the content of N may be fixedwith reference to respective contents of Ti, Al, and the like.Meanwhile, an excessive content of N possibly increases manufacturingcosts due to an increase in the respective contents of Ti, Al, and thelike, so that an upper limit of the content of N is preferably 0.01%.

One of or two or more of Ti: not less than 0.005% nor more than 0.1%, B:not less than 0.0003% nor more than 0.01%, Cr: not less than 0.01% normore than 1.0%, Ni: not less than 0.01% nor more than 5.0%, Mo: not lessthan 0.005% nor more than 2.0%, and Cu: not less than 0.005% nor morethan 1.0%

Ti: not less than 0.005% nor more than 0.1%

Titanium (Ti) is not only a reinforcing element for the base 101 butalso an element capable of improving the heat resistance of theAl-plating layer 103 formed on the surface of the base 101. Ti containedin an amount of less than 0.005% fails to sufficiently improve thestrength and heat resistance. Meanwhile, Ti added in an excessive amountwould form, for instance, carbide or nitride, softening the base 101. Inparticular, Ti contained in an amount exceeding 0.1% is highly unlikelyto achieve the desired mechanical strength. Accordingly, Ti is morepreferably added in an amount of 0.005% to 0.1%, both inclusive. Itshould be noted that the content of Ti is further preferably in a rangefrom 0.03% to 0.08%, both inclusive.

B: not less than 0.0003% nor more than 0.01%

Boron (B) is an element that works during quenching, exhibiting aneffect in improving the strength of the base 101.

B contained in an amount of less than 0.0003% fails to exhibit asufficient effect in improving the strength. Meanwhile, B contained inan amount exceeding 0.01% would form an inclusion (e.g., BN and carbonboride) in the base 101, increasing the shortness and, consequently,lowering fatigue strength. Accordingly,

B is more preferably added in an amount of 0.0003% to 0.01%, bothinclusive. It should be noted that the content of B is furtherpreferably in a range from 0.001% to 0.008%, both inclusive.

Cr: not less than 0.01% nor more than 1.0%

Chrome (Cr) has an effect in reducing generation of AlN, which causesseparation of the Al-plating layer 103, in an interface between theAl-plating layer 103 and the base 101 when the Al-plating layer 103 isalloyed to form an Al—Fe alloy layer. Additionally, Cr is not only anelement capable of improving wear resistance but also an element capableof enhancing hardenability. Cr contained in an amount of less than 0.01%fails to sufficiently exhibit the above effects. Meanwhile, Cr containedin an amount exceeding 1.0% not only saturates the above effects butalso increases the manufacturing costs of the steel sheet. Accordingly,Cr is more preferably added in an amount of 0.01% to 1.0%, bothinclusive. It should be noted that the content of Cr is furtherpreferably in a range from 0.5% to 1.0%, both inclusive.

Ni: not less than 0.01% nor more than 5.0%

Nickel (Ni) has an effect in improving hardenability during hotpressing. Ni also has an effect in enhancing the corrosion resistance ofthe base 101. However, Ni contained in an amount of less than 0.01%fails to sufficiently exhibit the above effects. Meanwhile, Ni containedin an amount exceeding 5.0% not only saturates the above effects butalso increases the manufacturing costs of the steel sheet. Accordingly,Ni is more preferably added in an amount of 0.01% to 5.0%, bothinclusive.

Mo: not less than 0.005% nor more than 2.0%

Molybdenum (Mo) has an effect in improving hardenability during hotpressing. Mo also has an effect in enhancing the corrosion resistance ofthe base 101. However, Mo contained in an amount of less than 0.005%fails to sufficiently exhibit the above effects. Meanwhile, Mo containedin an amount exceeding 2.0% not only saturates the above effects butalso increases the manufacturing costs of the steel sheet. Accordingly,Mo is more preferably added in an amount of 0.005% to 2.0%, bothinclusive.

Cu: not less than 0.005% nor more than 1.0%

Copper (Cu) has an effect in improving hardenability during hotpressing. Cu also has an effect in enhancing the corrosion resistance ofthe base 101. Cu contained in an amount of less than 0.005% fails tosufficiently exhibit the above effects. Meanwhile, Cu contained in anamount exceeding 1.0% not only saturates the above effects but alsoincreases the manufacturing costs of the steel sheet. Accordingly, Cu ismore preferably added in an amount of 0.005% to 1.0%, both inclusive.

W, V, Nb, and Sb

It should be noted that in addition to the above plurality of elements,elements such as tungsten (W), vanadium (V), niobium (Nb), antimony (Sb)may be selectively added to the above base 101 according to theexemplary embodiment. These elements may each be added in any content ina known range.

Balance

The balance of the base 101 consists of iron (Fe) and inevitableimpurities. The inevitable impurities include components inherentlypresent in a material and components naturally mixed during themanufacturing process, which are not deliberately contained in the base101.

The base 101 containing the above components exhibits a mechanicalstrength of approximately 1500 MPa or more after quenched by heating forhot pressing or the like. In spite of such an excellent mechanicalstrength, the steel sheet can be easily press-formed by hot pressing asthe steel sheet is thermally softened. Furthermore, when cooled from ahigh temperature after pressed, the base 101 exhibits a high mechanicalstrength. The mechanical strength can be maintained or improved evenwhen the thickness is reduced for weight reduction.

Al-Plating Layer 103

The Al-plating layer 103 is formed on at least one of opposite surfacesof the base 101. For instance, the Al-plating layer 103 is formed by,but not limited to, hot dip coating. The Al-plating layer 103 is notlimited as long as it consists mainly of Al. The wording “consistingmainly of Al” herein means that Al is contained in an amount of 50 mass% or more. The content of Al (the main component) is preferably 70 mass% or more, which means that Al-plating layer 103 preferably contains Alin an amount of 70 mass % or more. Components other than Al are notlimited but Si may be contained at a predetermined concentration.

The Al-plating layer 103 is configured to prevent corrosion of the base101. The Al-plating layer 103 is also configured to prevent scale (oxideof iron) from being generated on the steel surface during preheating forhot pressing. Thus, the presence of the Al-plating layer 103 on at leastone of opposite surfaces of the base 101 can eliminate the necessity ofprocesses such as descaling, surface cleaning, and surface treatmentand, consequently, improves the productivity of automobile parts and thelike. Furthermore, since a melting point of the Al-plating layer 103 ishigher than that of the metallic coating of an organic material or anyother metallic material (e.g., Zn material), the Al-plating layer 103can be processed at a high temperature during hot pressing.

It should be noted that Al contained in the Al-plating layer 103 issometimes partly or fully alloyed with Fe in the base 101 during hot dipcoating or hot pressing. Thus, the Al-plating layer 103 is not always inthe form of a single layer with fixed components. For instance, theAl-plating layer 103 sometimes includes a partially alloyed layer (alloylayer) or a steel-aluminum gradient alloy layer with variation inconcentration gradient from the surface thereof.

Surface Layer 107

The surface layer 107 is formed on the Al-plating layer 103. The surfacelayer 107 contains the ZnO particles 109 with a mean particle size in arange from 0.10 μm to 5.00 μm, both inclusive, and the organic resin111. An amount of coating of the ZnO particles 109 needs to be in arange from 0.5 g/m² to 10.0 g/m², both inclusive, in terms of metal Zn.It should be noted that for the configuration where the Al-plating layer103 is formed on each of the opposite surfaces of the base 101, thesurface layer 107 may be formed on the Al-plating layer 103 on at leastone of the opposite surfaces.

The surface layer 107 may be formed using, for instance, a solutionprepared by blending the above components in a variety of solvents suchas water and organic solvent.

ZnO Particles 109

To achieve good formability and corrosion resistance during hotpressing, the ZnO particles 109 with a mean particle size of 0.10 μm to5.00 μm, both inclusive, need to be formed on the Al-plating layer 103in an amount of coating of 0.5 g/m² to 10.0 g/m², both inclusive, interms of metal Zn. The ZnO particles 109 come into point-contact withthe mold with a lowered kinematic friction coefficient, thus improvingformability. However, the ZnO particles 109 with a mean particle size ofless than 0.10 μm fail to sufficiently improve formability due to anexcessive number of contact points between the ZnO particles 109 and themold.

Meanwhile, the ZnO particles 109 with a mean particle size exceeding5.00 μm lowers weldability. In spite of the insulating properties ofZnO, the ZnO particles 109 with a small particle size are crushed uponapplication of a welding pressure, ensuring sufficient powerdistribution points. However, when the ZnO particles 109 have a largemean particle size of more than 5 μm, the ZnO particles 109 are unlikelyto be crushed upon application of a welding pressure. As a result,sufficient power distribution points cannot be ensured, so that dust iseasily caused to lower weldability.

It should be noted that a method of determining the mean particle sizeof the ZnO particles 109 is not limited. For instance, the mean particlesize may be determined by: observing any selected ten or more of the ZnOparticles 109 at 2000-fold magnification with a SEM (Scanning ElectronMicroscope) or the like; and measuring and averaging maximum particlesizes of these particles. Alternatively, the mean particle size of theZnO particles 109 may be determined using a particle size distributionmeasuring device.

When the amount of coating of all the ZnO particles 109 in the surfacelayer 107 is less than 0.5 g/m² in terms of metal Zn, sufficientformability cannot be exhibited during hot pressing. Meanwhile, with anincrease in the amount of coating of the ZnO particles 109, a filmelectrical resistance of the surface layer 107 increases to lowerresistance weldability (e.g., spot weldability) of the steel sheet,although formability and corrosion resistance increase. In other words,when the amount of coating of all the ZnO particles 109 in the surfacelayer 107 exceeds 10.0 g/m² in terms of metal Zn, a sufficientresistance weldability of the steel sheet is difficult to achieve.

It should be noted that the amount of coating of the ZnO particles 109on the Al-plating layer 103 may be measured by a calibration curvemethod using XRF (X-ray Fluorescence).

It should be noted that the term “amount of coating” herein means anamount of coating measured before the steel sheet is set and heated onthe conveyor for hot pressing.

Organic Resin 111

For the steel sheet according to the exemplary embodiment, the organicresin 111, which is a component of the surface layer 107, is not limitedas long as the organic resin 111 functions as a binder capable ofkeeping the ZnO particles 109 within the metallic coating. The organicresin 111 is configured be combusted to disappear during preheating forhot pressing, so that the subsequent processes, such as pressing andwelding, are performed without any influence thereof. The organic resin111 may be an aqueous chemical agent. In this case, for instance, acation resin, which is mildly alkaline and stable as well as ZnO, ispreferably usable and examples of the cation resin include cationicurethane resin and cationic acrylic resin. It should be noted that aratio of the concentration (g/kg) of the organic resin in the chemicalagent is not limited according to the exemplary embodiment. Exemplaryresins usable as the organic resin 111 according to the exemplaryembodiment of the invention include a cationic urethane resin(manufactured by DKS Co. Ltd., trade name: SUPERFLEX 650).

To allow the organic resin 111 to sufficiently function as a binder, thecontent of the organic resin 111 with respect to the surface layer 107as a whole is preferably in a range from, in mass %, 10% to 60%, bothinclusive. At a content of less than 10%, the organic resin 111 fails tosufficiently function as a binder, making the metallic coating easy toseparate before preheating. It should be noted that the content of theorganic resin 111 is preferably 15% or more so that the organic resin111 stably functions as a binder. Meanwhile, a content of the organicresin 111 exceeding 60% results in unignorable emission of unpleasantodor.

Acetylacetonato

For the steel sheet according to the exemplary embodiment, the surfacelayer 107, especially, acetylacetonato contained as an oxidizer foroxidizing Al in the surface layer 107 is considerably important. Duringcontinuous heating, the contact area of the Al-plated steel sheet 100with the conveyor suffers from oxidation of the organic resin 111 and Alin the plating surface, which is accompanied by reduction of ZnO tometal Zn and, consequently, vaporization of the metal Zn. However,addition of acetylacetonato, which is more reducible (i.e., moreunlikely to be oxidized) than ZnO, to the surface layer 107 reduces theabove behavior of ZnO and, consequently, reduces the loss of ZnO. Inother words, addition of acetylacetonato to the surface layer 107reduces the loss of ZnO contributable to lubricity, allowing for stableformability during hot pressing and corrosion resistance.

Acetylacetonato may be added by itself or added in the form of anacetylacetonato complex (acetylacetonato metal salt). Examples of theacetylacetonato complex include copper complex, manganese complex,nickel complex, zinc complex, titanium complex, and vanadyl complex.Among the above, zinc acetylacetonato (zinc complex) is preferable,since it functions by itself as a ZnO source that generates ZnO.

When the content of acetylacetonato accounts for less than 10 mass % ofthe entire surface layer 107, which contains ZnO and acetylacetonato, interms of acetylacetonato, the reduction and loss of ZnO cannot besufficiently reduced. As a result, lubricity for hot working is lowered.Furthermore, when acetylacetonato is contained in a small amount, theamount of zinc oxide is reduced, thus lowering corrosion resistance ofpainted steel. Meanwhile, in an amount exceeding 30 mass %,acetylacetonato, which is soluble, makes film adhesion worse, loweringcorrosion resistance of painted steel.

It should be noted that the amount in terms of acetylacetonato isdetermined as follows. First, a predetermined area of the surface layer107 is removed using fuming nitric acid, and a weight of the surfacelayer 107 is measured before and after the removal to calculate ametallic coating amount of coating. Subsequently, the removed metalliccoating is well stirred in a known amount of a 30-degrees-C. solution toelute acetylacetonato contained in the metallic coating, a concentrationof the acetylacetonato in the solution is measured by a calibrationcurve method based on liquid chromatography, and an amount of theacetylacetonato contained in the metallic coating is calculated from theamount of the solution, while a ratio of the acetylacetonato iscalculated from the metallic coating amount of coating.

The surface layer 107 may be formed on the Al-plating layer 103 by, butnot limited to, a method including: preparing a solution or solventwhere the above main components (i.e., ZnO particles 109, organic resin111, and acetylacetonato) are dissolved; applying this solution orsolvent on the Al-plating layer 103 using a known device such as rollcoater and spray; and drying the applied solution or solvent. Theapplied solution or solvent may be dried by, but not limited to, avariety of techniques such as hot-air heating, IH (Induction Heating),NIR (Near InfraRed) heating, and resistance heating. Moreover, a heatingtemperature for drying is preferably determined as desired, consideringa glass transition temperature (Tg) of the organic resin 111 (binder).

As described above, the Al-plated steel sheet 100 according to theexemplary embodiment exhibits an excellent lubricity during hot pressingwithout loss of ZnO metallic coating at the contact area of the surfacelayer 107 (outermost layer) with the conveyor, achieving, for instance,excellent formability during hot pressing and excellent corrosionresistance after hot pressing. Furthermore, the Al-plated steel sheet100 according to the exemplary embodiment is less adhesive to a mold dueto the presence of the highly lubricative surface layer 107. If theAl-plating layer 103 is accidentally powdered by heating, adhesion ofthe powder (e.g., Al—Fe powder) to a mold used for subsequent pressingwould be reduced by the presence of the highly lubricative surface layer107. Thus, the Al-plated steel sheet 100 according to the exemplaryembodiment can be hot-pressed without the necessity of removal of Al—Fepowder adhering to the mold, achieving excellent productivity.

EXAMPLE(S)

Effects of the exemplary embodiment of the invention will bespecifically described below with reference to Examples. It should benoted that the conditions for Examples of the invention below are notexhaustive.

A cold-rolled steel sheet (balance: Fe and inevitable impurities,thickness: 1.4 mm) with chemical components shown in Table 1 was used.The Al-plating layer 103 was formed on each of opposite surfaces of thecold-rolled steel sheet by Sendzimir process. An annealing temperaturefor forming the Al-plating layer 103 was approximately 800 degrees C. Atypical Al-plating bath was used.

It should be noted that an amount of the Al-plating layer 103 adheringto the base 101 was adjusted to 160 g/m² per each surface by gas wiping.

TABLE 1 C Si Mn P S Ti B Al N 0.22 0.12 1.25 0.01 0.005 0.02 0.003 0.040.0005

Subsequently, the surface layer 107 was formed by applying on theAl-plating layer 103 a solution prepared by blending a dispersant of theZnO particles 109 (manufactured by SAKAI CHEMICAL INDUSTRY CO., LTD.,trade name: DIF-3ST4S), a cationic urethane resin (manufactured by DKSCo. Ltd., trade name: SUPERFLEX 650) as the organic resin 111, andacetylacetonato using a roll coater, and drying the solution such that asheet temperature reached 80 degrees C. For Sample No. 17, the organicresin 111 was not used and cyclopentasiloxane with ZnO particles 109being dispersed was applied and then dried at 80 degrees C.

Sample Al-plated steel sheets were thus produced. It should be notedthat the solution for forming the surface layer 107 shown in Table 2 wasprepared by blending a commercially available reagent with distilledwater.

It should also be noted that the term “Zn Amount of Coating” in Table 2means a value (unit: g/m²) of a total amount of coating of the ZnOparticles 109 per square meter in terms of the mass of metal Zn, thevalue being measured by a calibration curve method using XRF.

Measurement Conditions

The content of acetylacetonato, the particle size of the ZnO particles109, and the ZnO amount of coating were identified as follows.

1. Content of Acetylacetonato

The surface layer 107 was partly removed using fuming nitric acid asdescribed above and a concentration of the removed acetylacetonato wasmeasured by a calibration curve method based on liquid chromatography.

2. Particle Size of ZnO Particles 109

The ZnO particles 109 were observed at 2000-fold magnification with ascanning electron microscope manufactured by JEOL Ltd. (trade name:JSM-7800F) and respective maximum particle sizes of 20 of the ZnOparticles 109 were measured and averaged. The average value was definedas the particle size of the ZnO particles 109.

3. ZnO Amount of Coating

The ZnO amount of coating was measured with an X-ray fluorescenceanalyzer manufactured by Rigaku Corporation (trade name: ZSX Primus)under the following conditions. For the measurement, an analytical curvewas created in advance that represented a relationship between ZnOamount of coating and intensity of X-ray fluorescence in terms of metalZn content and the amount of coating was determined with reference tothe analytical curve.

measurement diameter: 30 mm

measurement atmosphere: vacuum

spectrum: Zn—Kα

The number of counted peaks of Zn—Kα of X-ray fluorescence analysismeasured under the above conditions was referred to.

TABLE 2 Outer Cladding Layer 107 Organic Resin 111 ZnO Ratio of OxidizerParticles 109 Organic Resin Content Particle Zn Amount in in Terms ofSize of Coating Chemical Agent Acetylacetonato (μm) (g/m²) Resin Species(mass %) Type (mass %) Sample No. 1 0.1 1.5 cationic urethane resin 30acetylacetonato 20 Sample No. 2 1 1.5 cationic urethane resin 30acetylacetonato 20 Sample No. 3 5 1.5 cationic urethane resin 30acetylacetonato 20 Sample No. 4 1 0.5 cationic urethane resin 30acetylacetonato 20 Sample No. 5 1 10 cationic urethane resin 30acetylacetonato 20 Sample No. 6 1 1.5 cationic urethane resin 30acetylacetonato 30 Sample No. 7 1 1.5 cationic urethane resin 30acetylacetonato 10 Sample No. 8 1 1.5 cationic urethane resin 60acetylacetonato 20 Sample No. 9 1 1.5 cationic urethane resin 10acetylacetonato 20 Sample No. 10 1 1.5 cationic acrylic resin 30acetylacetonato 20 Sample No. 11 0.08 1.5 cationic urethane resin 30acetylacetonato 20 Sample No. 12 7 1.5 cationic urethane resin 30acetylacetonato 20 Sample No. 13 1 0.3 cationic urethane resin 30acetylacetonato 20 Sample No. 14 1 12   cationic urethane resin 30acetylacetonato 20 Sample No. 15 1 1.5 cationic urethane resin 30acetylacetonato  5 Sample No. 16 1 1.5 cationic urethane resin 30acetylacetonato 35 Sample No. 17 1 1.5 None  0 acetylacetonato 20 SampleNo. 18 1 1.5 cationic urethane resin 30 None  0

Next, the sample steel sheets produced as described above were evaluatedby the following methods in terms of a variety of properties. Table 3shows the results.

(1) Lubricity for Hot Working (Lubricity)

For evaluation of lubricity for hot working, each sample steel sheet wassubjected to a hot mold-pulling test. More specifically, each 30 mm×350mm sample steel sheet was put in a furnace and heated at 900 degrees C.for 6 minutes while sandwiched between two SiC plates (60 mm width×200mm length×30 mm thickness) and taken out of the furnace. A flat mold (50mm width×40 mm length) of SKD11 was then pressed against oppositesurfaces of the steel sheet at approximately 700 degrees C. for apulling process. The steel sheet was sandwiched between by the SiCplates at both sides thereof with supply of oxygen through the surfacesbeing sufficiently cut in order to simulate the situation where the ZnOmetallic coating disappeared at the stacked portion in contact with theconveyor under more severer conditions. A pressing load and a pullingload were measured and a value calculated by pulling load/(2×pressingload) was defined as a hot friction coefficient. It should be noted thata smaller kinematic friction coefficient means a higher lubricity forhot working, and a kinematic friction coefficient of less than 0.52 isevaluated to pass in Table 3.

(2) Corrosion Resistance of Painted Steel

Each 120 mm×200 mm sample steel sheet was put in a furnace and placed onan in-furnace SiC mount with an evaluation surface of the steel sheet incontact with the mount. The steel sheet was then heated in the furnaceat 900 degrees C. for 6 minutes with a SUS304 block (50 mm×50 mm×70 mm)having been heated to 900 degrees C. placed thereon. Immediately aftertaken out of the furnace, the steel sheet was sandwiched between astainless steel mold for rapid cooling. The cooling rate wasapproximately 150 degrees C./second. Next, each steel sheet having beencooled was cut from a center thereof into a 70 mm×150 mm piece. Aftersubjected to a chemical conversion treatment with a chemical conversionsolution (PB-SX35) manufactured by Nihon Parkerizing Co., Ltd., thepiece was coated with a electrodeposition paint (POWERNICS 110)manufactured by Nippon Paint Co., Ltd. such that a thickness of thepiece reached 20 μm, and burnt at 170 degrees C. It should be noted thateach 70 mm×150 mm steel sheet with a thermocouple welded thereto was putin an air atmosphere furnace whose temperature was set to 900 degreesC., and a temperature of the steel sheet was measured until it reached900 degrees C. to calculate an average temperature-rise rate. Theaverage temperature-rise rate was 5 degrees C./second.

Evaluation of corrosion resistance of painted steel was performed by amethod in accordance with JASO M609 instituted by Society of AutomotiveEngineers of Japan, Inc. Specifically, a film was cross-cut in advancewith a cutter, and a width (maximum value on one side) of swelling ofthe film from the cross cut was measured after the elapse of 180 cyclesof a corrosion test (60 days). A smaller width of swelling of the filmmeans a higher corrosion resistance and a width equal to or less than 5mm is evaluated to pass in Table 3.

(3) Spot Weldability

Spot weldability was evaluated as follows.

The prepared sample steel sheets were each put in a furnace and heatedtherein at 900 degrees for 6 minutes. The sample steel sheets were theneach sandwiched by a stainless steel mold for rapid cooling immediatelyafter taken out of the furnace. The cooling rate was approximately 150degrees C./second. Each cooled steel sheet was cut into a 30×50 mm piecefor measurement of a suitable current range for spot welding (maximumcurrent to minimum current). The measurement conditions are as follows.A current value achieving a nugget diameter of 3×(t)0.5 was defined asthe minimum current, whereas a current causing expulsion was defined asthe maximum current.

current: DC

electrode: made of chrome copper, DR (40R/6-mm-diameter tip end)

compression: 400 kgf (1 kgf=9.8N)

power supply time: 240 microseconds

A larger value means a higher spot weldability and a spot weldability of1.0 kA or more is evaluated to pass in Table 3.

(4) ZnO Disappearance Test

Each sample steel sheet was punched into a diameter of 30 mm, andstacked on a 70 mm×70 mm in-furnace SiC mount while a 50 mm×50 mm×70 mmSUS304 block having been heated to 900 degrees C. was placed thereon.The steel sheet in this state was then heated in a furnace at 900degrees C. for 6 minutes, and sandwiched by a stainless steel mold forrapid cooling immediately after taken out of the furnace. Values of Znamount of coating before and after the heating were measured with XRF.The Zn amount of coating was measured in terms of Zn and a ZnO residualratio was calculated in terms of Zn.

A sample with a Zn residual ratio of 75% or more and a Zn residualamount of 0.40 g/m² or more is evaluated to pass in Table 3.

TABLE 3 ZnO Disappearance Test Zn Residual Zn Residual Corrosion RatioAmount Lubricity for Resistance of (%) (g/m²) Hot Working Painted SteelWeldability Sample No. 1 78 1.17 0.46 2.7 2.8 Sample No. 2 77 1.15 0.462.8 2.8 Sample No. 3 78 1.17 0.46 2.7 2.8 Sample No. 4 80 0.4 0.51 4.93.0 Sample No. 5 80 8 0.43 2.1 1.1 Sample No. 6 79 1.18 0.46 2.6 2.8Sample No. 7 75 1.13 0.46 2.9 2.8 Sample No. 8 77 1.16 0.46 2.8 2.8Sample No. 9 77 1.15 0.46 2.7 2.8 Sample No. 10 78 1.17 0.46 2.7 2.8Sample No. 11 78 1.17 0.52 2.7 2.8 Sample No. 12 79 1.19 0.50 2.7 0.7Sample No. 13 77 0.23 0.63 7.8 3.0 Sample No. 14 88 10.5 0.43 2 0.7Sample No. 15 23 0.34 0.55 7.1 2.8 Sample No. 16 84 1.26 0.46 5.6 2.8Sample No. 17 77 1.16 0.55 5.3 2.8 Sample No. 18 3 0.05 0.68 8 2.8

As is understood from Tables 2 and 3, it has been found that all of theAl-plated steel sheets for hot pressing within the scope of theinvention (sample Nos. 1 to 10) achieved excellent results of all thefour evaluations (the variety of properties and ZnO disappearance test),where: the surface layer 107 contained the organic resin 111; the meanparticle size of the ZnO particles 109 was in a range from 0.10 μm to5.00 μm (both inclusive); the amount of coating of the ZnO particles 109was in a range from 0.5 g/m² to 10.0 g/m² (both inclusive) in terms ofmetal Zn; and acetylacetonato was added to the surface layer 107 in anamount of 10% to 30% (both inclusive) in terms of acetylacetonato.

In contrast, it has been found that none of the Al-plated steel sheetsfor hot pressing out of the scope of the invention (sample Nos. 11 to18) achieved an excellent result of at least one of the above fourevaluations (the variety of properties and ZnO disappearance test),where the organic resin 111 was not contained, or one of the content ofacetylacetonato, the mean particle size of the ZnO particles 109, theamount of coating of the ZnO particles 109, and the like was out of thepredetermined range according to the invention.

EXPLANATION OF CODE(S)

100 . . . Al-plated steel sheet

101 . . . base

103 . . . Al-plating layer

107 . . . surface layer

109 . . . ZnO particles

111 . . . organic resin

1. An Al-plated steel sheet for hot pressing, comprising: a base; anAl-plating layer formed on at least one of opposite surfaces of thebase; and a surface layer formed on the Al-plating layer, the surfacelayer comprising: ZnO particles; an organic resin; and acetylacetonatoin an amount in a range from 10 mass % to 30 mass %, both inclusive,with respect to a total mass of the surface layer, wherein a meanparticle size of the ZnO particles is in a range from 0.10 μm to 5.00μm, both inclusive, and an amount of coating of the ZnO particles is ina range from 0.5 g/m² to 10.0 g/m², both inclusive, in terms of metalZn.
 2. The Al-plated steel sheet according to claim 1, wherein theAl-plating layer and the surface layer are formed on each of theopposite surfaces of the base.